Structural determinants of A(3) adenosine receptor activation: nucleoside ligands at the agonist/antagonist boundary

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

A3 Adenosine Receptor Antagonists with Nucleoside Structures and Their Anticancer Activity

The overexpression of the A3 adenosine receptor (AR) in a number of cancer cell types makes it an attractive target for tumor diagnosis and therapy. Hence, in the search for new A3AR ligands, a series of novel 2,N6-disubstituted adenosines (Ados) was synthesized and tested in radioligand binding and functional assays at ARs. Derivatives bearing a 2-phenethylamino group in the N6-position were found to exert higher A3AR affinity and selectivity than the corresponding N6-(2,2-diphenylethyl) analogues. 2-Chloro-N6-phenylethylAdo (15) was found to be a potent full A3AR agonist with a Ki of 0.024 nM and an EC50 of 14 nM, in a cAMP accumulation assay. Unlike 15, the other ligands behaved as A3AR antagonists, which concentration-dependently reduced cell growth and exerted cytostatic activity on the prostate cancer cell line PC3, showing comparable and even more pronounced effects with respect to the ones elicited by the reference full agonist Cl-IB-MECA. In particular, the N6-(2,2-diphenylethyl)-2-phenylethynylAdo (12: GI50 = 14 µM, TGI = 29 µM, and LC50 = 59 µM) showed the highest activity proving to be a potential antitumor agent. The cytostatic effect of both A3AR agonist (Cl-IB-MECA) and antagonists (12 and other newly synthesized compounds) confirm previous observations according to which, in addition to the involvement of A3ARs, other cellular mechanisms are responsible for the anticancer effects of these ligands.

Stereodivergent Synthesis of Biologically Active Spironucleoside Scaffolds via Catalytic Cyclopropanation of 4-exo-Methylene Furanosides

Cyclopropane fusion of the only rotatable carbon-carbon bond in furanosyl nucleosides (i.e., exocyclic 4'-5') is a powerful design strategy to arrive at conformationally constrained analogues. Herein, we report a direct stereodivergent route toward the synthesis of the four possible configurations of 4-spirocyclopropane furanoses, which have been transformed into the corresponding 4'-spirocyclic adenosine analogues. The latter showed differential inhibition of the protein methyltransferase PRMT5-MEP50 complex, with one analogue inhibiting more effectively than adenosine itself, demonstrating the utility of rationally probing 4'-5' side chain orientations.

Pharmacological characterization of DPTN and other selective A3 adenosine receptor antagonists

The A₃ adenosine receptor (AR) is emerging as an attractive drug target. Antagonists are proposed for the potential treatment of glaucoma and asthma. However, currently available A₃AR antagonists are potent in human and some large animals, but weak or inactive in mouse and rat. In this study, we re-synthesized a previously reported A₃AR antagonist, DPTN, and evaluated its affinity and selectivity at human, mouse, and rat ARs. We showed that DPTN, indeed, is a potent A₃AR antagonist for all three species tested, albeit a little less selective for mouse and rat A₃AR in comparison to the human A₃AR. DPTN's K_i values at respective A₁, A_2A, A_2B, and A₃ receptors were (nM) 162, 121, 230, and 1.65 (human); 411, 830, 189, and 9.61 (mouse); and 333, 1147, 163, and 8.53 (rat). Its antagonist activity at both human and mouse A₃ARs was confirmed in a cyclic AMP functional assay. Considering controversial use of currently commercially available A₃AR antagonists in rats and mice, we also re-examined other commonly used and selective A₃AR antagonists under the same experimental conditions. The K_i values of MRS1523 were shown to be 43.9, 349, and 216 nM at human, mouse, and rat A₃ARs, respectively. MRS1191 and MRS1334 showed incomplete inhibition of [¹²⁵I]I-AB-MECA binding to mouse and rat A₃ARs, while potent human A₃AR antagonists, MRS1220, MRE3008F20, PSB10, PSB-11, and VUF5574 were largely inactive. Thus, we demonstrated that DPTN and MRS1523 are among the only validated A₃AR antagonists that can be possibly used (at an appropriate concentration) in mouse or rat to confirm an A₃AR-related mechanism or function.

Subtle Chemical Changes Cross the Boundary between Agonist and Antagonist: New A3 Adenosine Receptor Homology Models and Structural Network Analysis Can Predict This Boundary

Distinguishing compounds' agonistic or antagonistic behavior would be of great utility for the rational discovery of selective modulators. We synthesized truncated nucleoside derivatives and discovered 6c (K_i = 2.40 nM) as a potent human A₃ adenosine receptor (hA₃AR) agonist, and subtle chemical modification induced a shift from antagonist to agonist. We elucidated this shift by developing new hA₃AR homology models that consider the pharmacological profiles of the ligands. Taken together with molecular dynamics (MD) simulation and three-dimensional (3D) structural network analysis of the receptor-ligand complex, the results indicated that the hydrogen bonding with Thr94^3.36 and His272^7.43 could make a stable interaction between the 3'-amino group with TM3 and TM7, and the corresponding induced-fit effects may play important roles in rendering the agonistic effect. Our results provide a more precise understanding of the compounds' actions at the atomic level and a rationale for the design of new drugs with specific pharmacological profiles.

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

Two New Adenosine Derivatives and their Antiproliferative Properties, an In Vitro Evaluation

BACKGROUND

Adenosine is a natural nucleoside present in various organs and tissues, where it acts as a modulator of diverse physiological and pathophysiological processes. These actions are mediated by at least four G protein-coupled receptors, which are widely and differentially expressed in tissues. Interestingly, high concentrations of adenosine have been reported in a variety of tumors. In this context, the final output of adenosine in tumorigenesis will likely depend on the constellation of adenosine receptors expressed by tumor and stromal cells. Notably, activation of the A3 receptor can reduce the proliferative capacity of various cancer cells.

OBJECTIVE

The objective of this study is to describe the anti-proliferative effects of two previously synthesized adenosine derivatives with A3 agonist action (compounds 2b and 2f) through in vitro assays.

METHOD

We used gastric and breast cancer cell lines expressing the A3 receptor as in vitro models and theoretical experiments such as molecular dynamics and determination of ADME properties.

RESULTS

The antiproliferative effects of adenosine derivatives (after determining IC50 values) were comparable or even higher than those described for IB-MECA, a commercially available A3 agonist. Among possible mechanisms involved, apoptosis was found to be induced in MCF-7 cells but not in AGS or MDA-MB-231 cells. Surprisingly, we were unable to observe cellular senescence induction upon treatment with compounds 2b and 2f in any of the cell lines studied, although we cannot rule out other forms of cell cycle exit at this point.

CONCLUSION

Both adenosine derivatives showed antiproliferative effects on gastric and breast cancer cell lines, and were able to induce apoptosis, at least in the MCF-7 cell line. Further studies will be necessary to unveil receptor specificity and mechanisms accounting for the antiproliferative properties of these novel semi-synthetic compounds.

Design and Synthesis of 2,6-Disubstituted-4'-Selenoadenosine-5'-N,N-Dimethyluronamide Derivatives as Human A3 Adenosine Receptor Antagonists

A new series of 4′-selenoadenosine-5′-N,N-dimethyluronamide derivatives as highly potent and selective human A₃ adenosine receptor (hA₃AR) antagonists, is described. The highly selective A₃AR agonists, 4′-selenoadenosine-5′-N-methyluronamides were successfully converted into selective antagonists by adding a second N-methyl group to the 5′-uronamide position. All the synthesized compounds showed medium to high binding affinity at the hA₃AR. Among the synthesized compounds, 2-H-N⁶-3-iodobenzylamine derivative 9f exhibited the highest binding affinity at hA₃AR. (K_i = 22.7 nM). The 2-H analogues generally showed better binding affinity than the 2-Cl analogues. The cAMP functional assay with 2-Cl-N⁶-3-iodobenzylamine derivative 9l demonstrated hA₃AR antagonist activity. A molecular modelling study suggests an important role of the hydrogen of 5′-uronamide as an essential hydrogen bonding donor for hA₃AR activation.

Sc(OTf)3 catalyzed [3 + 2]-annulation reaction of donor–acceptor aziridines with methylene exo-glycals: synthesis of chiral carbohydrate-spiro-heterocycles

A Sc(OTf)₃ catalyzed [3 + 2]-annulation reaction between D–A N-tosyl aziridines and methylene exo-glycals was developed for the synthesis of carbohydrate-spiro-heterocycles.

Nucleotide P2Y1 receptor agonists are in vitro and in vivo prodrugs of A1/A3 adenosine receptor agonists: implications for roles of P2Y1 and A1/A3 receptors in physiology and pathology

Rapid phosphoester hydrolysis of endogenous purine and pyrimidine nucleotides has challenged the characterization of the role of P2 receptors in physiology and pathology. Nucleotide phosphoester stabilization has been pursued on a number of medicinal chemistry fronts. We investigated the in vitro and in vivo stability and pharmacokinetics of prototypical nucleotide P2Y₁ receptor (P2Y₁R) agonists and antagonists. These included the riboside nucleotide agonist 2-methylthio-ADP and antagonist MRS2179, as well as agonist MRS2365 and antagonist MRS2500 containing constrained (N)-methanocarba rings, which were previously reported to form nucleotides that are more slowly hydrolyzed at the α-phosphoester compared with the ribosides. In vitro incubations in mouse and human plasma and blood demonstrated the rapid hydrolysis of these compounds to nucleoside metabolites. This metabolism was inhibited by EDTA to chelate divalent cations required by ectonucleotidases for nucleotide hydrolysis. This rapid hydrolysis was confirmed in vivo in mouse pharmacokinetic studies that demonstrate that MRS2365 is a prodrug of the nucleoside metabolite AST-004 (MRS4322). Furthermore, we demonstrate that the nucleoside metabolites of MRS2365 and 2-methylthio-ADP are adenosine receptor (AR) agonists, notably at A₃ and A₁ARs. In vivo efficacy of MRS2365 in murine models of traumatic brain injury and stroke can be attributed to AR activation by its nucleoside metabolite AST-004, rather than P2Y₁R activation. This research suggests the importance of reevaluation of previous in vitro and in vivo research of P2YRs and P2XRs as there is a potential that the pharmacology attributed to nucleotide agonists is due to AR activation by active nucleoside metabolites.

Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A3 Adenosine Receptors: Affinity Enhancement by N6-(2-Phenylethyl) Substitution

Dopamine-derived N6-substituents, compared to N6-(2-phenylethyl), in truncated (N)-methanocarba (bicyclo[3.1.0]hexyl) adenosines favored high A3 adenosine receptor (AR) affinity/selectivity, e.g., C2-phenylethynyl analogue 15 (MRS7591, Ki = 10.9/17.8 nM, at human/mouse A3AR). 15 was a partial agonist in vitro (hA3AR, cAMP inhibition, 31% Emax; mA3AR, [35S]GTP-γ-S binding, 16% Emax) and in vivo and also antagonized hA3AR in vitro. Distal H-bonding substitutions of the N6-(2-phenylethyl) moiety particularly enhanced mA3AR affinity by polar interactions with the extracellular loops, predicted using docking and molecular dynamics simulation with newly constructed mA3AR and hA3AR homology models. These hybrid models were based on an inactive antagonist-bound hA1AR structure for the upper part of TM2 and an agonist-bound hA2AAR structure for the remaining TM portions. These species-independent A3AR-selective nucleosides are low efficacy partial agonists and novel, nuanced modulators of the A3AR, a drug target of growing interest.

Assessment of biased agonism at the A3 adenosine receptor using β-arrestin and miniGαi recruitment assays

The A₃ adenosine receptor (A₃AR) is a G protein-coupled receptor that is involved in a wide variety of physiological and pathological processes, such as cancer. However, the use of compounds pharmacologically targeting this receptor remains limited in clinical practice, despite extensive efforts for compound synthesis. Moreover, the possible occurrence of biased agonism further complicates the interpretation of the functional characteristics of compounds. Hence the need for simple assays, which are comparable in terms of the used cell lines and read-out technique. We previously established a stable β-arrestin 2 (βarr2) bioassay, employing a simple, luminescent read-out via functional complementation of a split nanoluciferase enzyme. Here, we developed a complementary, new bioassay in which coupling of an engineered miniGα_i protein to activated A₃AR is monitored using a similar approach. Application of both bioassays for the concurrent determination of the potencies and efficacies of a set of 19 N⁶-substituted adenosine analogues not only allowed for the characterization of structure-activity relationships, but also for the quantification of biased agonism. Although a broad distribution in potency and efficacy values was obtained within the test panel, no significant bias was observed toward either the βarr2 or miniGα_i pathway.

Design, Synthesis and Evaluation of New Indolylpyrimidylpiperazines for Gastrointestinal Cancer Therapy

Human A3 adenosine receptor hA3AR has been implicated in gastrointestinal cancer, where its cellular expression has been found increased, thus suggesting its potential as a molecular target for novel anticancer compounds. Observation made in our previous work indicated the importance of the carbonyl group of amide in the indolylpyrimidylpiperazine (IPP) for its human A2A adenosine receptor (hA2AAR) subtype binding selectivity over the other AR subtypes. Taking this observation into account, we structurally modified an indolylpyrimidylpiperazine (IPP) scaffold, 1 (a non-selective adenosine receptors' ligand) into a modified IPP (mIPP) scaffold by switching the position of the carbonyl group, resulting in the formation of both ketone and tertiary amine groups in the new scaffold. Results showed that such modification diminished the A2A activity and instead conferred hA3AR agonistic activity. Among the new mIPP derivatives (3-6), compound 4 showed potential as a hA3AR partial agonist, with an Emax of 30% and EC50 of 2.89 ± 0.55 μM. In the cytotoxicity assays, compound 4 also exhibited higher cytotoxicity against both colorectal and liver cancer cells as compared to normal cells. Overall, this new series of compounds provide a promising starting point for further development of potent and selective hA3AR partial agonists for the treatment of gastrointestinal cancers.

Structural Characterization of Agonist Binding to an A3 Adenosine Receptor through Biomolecular Simulations and Mutagenesis Experiments

The adenosine A₃ receptor (A₃R) binds adenosine and is a drug target against cancer cell proliferation. Currently, there is no experimental structure of A₃R. Here, we have generated a molecular model of A₃R in complex with two agonists, the nonselective 1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-β-d-ribofuranuronamide (NECA) and the selective 1-deoxy-1-[6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-N-methyl-β-d-ribofuranuronamide (IB-MECA). Molecular dynamics simulations of the wild-type A₃R in complex with both agonists, combined with in vitro mutagenic studies revealed important residues for binding. Further, molecular mechanics-generalized Born surface area calculations were able to distinguish mutations that reduce or negate agonistic activity from those that maintained or increased the activity. Our studies reveal that selectivity of IB-MECA toward A₃R requires not only direct interactions with residues within the orthosteric binding area but also with remote residues. Although V169^5.30 is considered to be a selectivity filter for A₃R binders, when it was mutated to glutamic acid or alanine, the activity of IB-MECA increased by making new van der Waals contacts with TM5. This result may have implications in the design of new A₃R agonists.

Semisynthetic aurones inhibit tubulin polymerization at the colchicine-binding site and repress PC-3 tumor xenografts in nude mice and myc-induced T-ALL in zebrafish

Structure-activity relationships (SAR) in the aurone pharmacophore identified heterocyclic variants of the (Z)-2-benzylidene-6-hydroxybenzofuran-3(2H)-one scaffold that possessed low nanomolar in vitro potency in cell proliferation assays using various cancer cell lines, in vivo potency in prostate cancer PC-3 xenograft and zebrafish models, selectivity for the colchicine-binding site on tubulin, and absence of appreciable toxicity. Among the leading, biologically active analogs were (Z)-2-((2-((1-ethyl-5-methoxy-1H-indol-3-yl)methylene)-3-oxo-2,3-dihydrobenzofuran-6-yl)oxy)acetonitrile (5a) and (Z)-6-((2,6-dichlorobenzyl)oxy)-2-(pyridin-4-ylmethylene)benzofuran-3(2H)-one (5b) that inhibited in vitro PC-3 prostate cancer cell proliferation with IC50 values below 100 nM. A xenograft study in nude mice using 10 mg/kg of 5a had no effect on mice weight, and aurone 5a did not inhibit, as desired, the human ether-à-go-go-related (hERG) potassium channel. Cell cycle arrest data, comparisons of the inhibition of cancer cell proliferation by aurones and known antineoplastic agents, and in vitro inhibition of tubulin polymerization indicated that aurone 5a disrupted tubulin dynamics. Based on molecular docking and confirmed by liquid chromatography-electrospray ionization-tandem mass spectrometry studies, aurone 5a targets the colchicine-binding site on tubulin. In addition to solid tumors, aurones 5a and 5b strongly inhibited in vitro a panel of human leukemia cancer cell lines and the in vivo myc-induced T cell acute lymphoblastic leukemia (T-ALL) in a zebrafish model.

Adenosine from a biologic source regulates neutrophil extracellular traps (NETs)

Neutrophil extracellular traps (NETs) are implicated in autoimmune, thrombotic, malignant, and inflammatory diseases; however, little is known of their endogenous regulation under basal conditions. Inflammatory effects of neutrophils are modulated by extracellular purines such as adenosine (ADO) that is inhibitory or ATP that generally up-regulates effector functions. In order to evaluate the effects of ADO on NETs, human neutrophils were isolated from peripheral venous blood from healthy donors and stimulated to make NETs. Treatment with ADO inhibited NET production as quantified by 2 methods: SYTOX green fluorescence and human neutrophil elastase (HNE)-DNA ELISA assay. Specific ADO receptor agonist and antagonist were tested for their effects on NET production. The ADO _2A receptor (A_2A R) agonist CSG21680 inhibited NETs to a similar degree as ADO, whereas the A_2A R antagonist ZM241385 prevented ADO's NET-inhibitory effects. Additionally, CD73 is a membrane bound ectonucleotidase expressed on mesenchymal stromal cells (MSCs) that allows manipulation of extracellular purines in tissues such as bone marrow. The effects of MSCs on NET formation were evaluated in coculture. MSCs reduced NET formation in a CD73-dependent manner. These results imply that extracellular purine balance may locally regulate NETosis and may be actively modulated by stromal cells to maintain tissue homeostasis.

Probing structure-activity relationship in β-arrestin2 recruitment of diversely substituted adenosine derivatives

In the adenosine receptor (AR) subfamily of G protein-coupled receptors (GPCRs), biased agonism has been described for the human A₁AR, A_2BAR and A₃AR. While diverse A₃AR agonists have been evaluated for receptor binding and G_i-mediated cAMP signalling, the β-arrestin2 (βarr2) pathway has been left largely unexplored. We screened nineteen diverse adenosine derivatives for βarr2 recruitment using a stable hA₃AR-NanoBit^®-βarr2 HEK293T cell line. Their activity profiles were compared with a cAMP accumulation assay in stable hA₃AR CHO cells. Structural features linked to βarr2 activation were further investigated by the evaluation of an additional ten A₃AR ligands. The A₃AR-selective reference agonist 2-Cl-IB-MECA, which is a full agonist in terms of cAMP inhibition, only showed partial agonist behaviour in βarr2 recruitment. Highly A₃AR-selective (N)-methanocarba 5'-uronamide adenosine derivatives displayed higher potency in both cAMP signalling and βarr2 recruitment than reference agonists NECA and 2-Cl-IB-MECA. Their A₃AR-preferred conformation tolerates C2-position substitutions, for increased βarr2 efficacy, better than the flexible scaffolds of ribose derivatives. The different amino functionalities in the adenosine scaffold of these derivatives each seem to be important for signalling as well. In conclusion, we have provided insights into ligand features that can help to guide the future therapeutic development of biased A₃AR ligands with respect to G-protein and βarr2 signalling.

Breakthrough in GPCR Crystallography and Its Impact on Computer-Aided Drug Design

Recent crystallographic structures of G protein-coupled receptors (GPCRs) have greatly advanced our understanding of the recognition of their diverse agonist and antagonist ligands. We illustrate here how this applies to A_2A adenosine receptors (ARs) and to P2Y₁ and P2Y₁₂ receptors (P2YRs) for ADP. These X-ray structures have impacted the medicinal chemistry aimed at discovering new ligands for these two receptor families, including receptors that have not yet been crystallized but are closely related to the known structures. In this Chapter, we discuss recent structure-based drug design projects that led to the discovery of: (a) novel A₃AR agonists based on a highly rigidified (N)-methanocarba scaffold for the treatment of chronic neuropathic pain and other conditions, (b) fluorescent probes of the ARs and P2Y₁₄R, as chemical tools for structural probing of these GPCRs and for improving assay capabilities, and (c) new more drug-like antagonists of the inflammation-related P2Y₁₄R. We also describe the computationally enabled molecular recognition of positive (for A₃AR) and negative (P2Y₁R) allosteric modulators that in some cases are shown to be consistent with structure-activity relationship (SAR) data. Thus, computational modeling has become an essential tool for the design of purine receptor ligands.

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

Semisynthesis, Characterization and Evaluation of New Adenosine Derivatives as Antiproliferative Agents

We describe the semisynthesis and biological effects of adenosine derivatives, which were anticipated to function as agonists for the A₃ receptor. Molecular docking was used to select candidate compounds. Fifteen nucleoside derivatives were obtained through nucleophilic substitutions of the N⁶-position of the nucleoside precursor 6-chloropurine riboside by amines of different origin. All compounds were purified by column chromatography and further characterized by spectroscopic and spectrometric techniques, showing moderate yield. These molecules were then evaluated for their antiproliferative activity in human gastric cancer cells expressing the A₃ receptor. We found that the compounds obtained have antiproliferative activity and that new structural modifications can enhance their biological activity. The ADME (Absorption, Distribution, Metabolism and Excretion) properties of the most active compounds were also evaluated theoretically.

GCDB: a glaucomatous chemogenomics database for in silico drug discovery

Glaucoma is a group of neurodegenerative diseases that can cause irreversible blindness. The current medications, which mainly reduce intraocular pressure to slow the progression of disease, may have local and systemic side effects. Recently, medications with possible neuroprotective effects have attracted much attention. To assist in the identification of new glaucoma drugs, we created a glaucomatous chemogenomics database (GCDB; http://cadd.pharmacy.nankai.edu.cn/gcdb/home) in which various glaucoma-related chemogenomics data records are assembled, including 275 genes, 105 proteins, 83 approved or clinical trial drugs, 90 206 chemicals associated with 213 093 records of reported bioactivities from 22 324 corresponding bioassays and 5630 references. Moreover, an improved chemical similarity ensemble approach computational algorithm was incorporated in the GCDB to identify new targets and design new drugs. Further, we demonstrated the application of GCDB in a case study screening two chemical libraries, Maybridge and Specs, to identify interactions between small molecules and glaucoma-related proteins. Finally, six and four compounds were selected from the final hits for in vitro human glucocorticoid receptor (hGR) and adenosine A3 receptor (A3AR) inhibitory assays, respectively. Of these compounds, six were shown to have inhibitory activities against hGR, with IC50 values ranging from 2.92-28.43 μM, whereas one compoundshowed inhibitory activity against A3AR, with an IC50 of 6.15 μM. Overall, GCDB will be helpful in target identification and glaucoma chemogenomics data exchange and sharing, and facilitate drug discovery for glaucoma treatment.

Two-Step, One-Pot Synthesis of Visible-Light-Responsive 6-Azopurines

The first general two-step, one-pot synthetic route to 6-azopurines is presented. Microwave-assisted nucleophilic aromatic substitution of protected 6-chloropurines with hydrazines or hydrazides, followed by metal-free oxidation with oxygen, gives 6-azopurines in high to excellent yields. Photophysical studies revealed intensive n-π* absorption band that makes trans-to-cis photoswitching possible using visible light (λ = 530 nm).

Structural Probing and Molecular Modeling of the A₃ Adenosine Receptor: A Focus on Agonist Binding

Adenosine is an endogenous modulator exerting its functions through the activation of four adenosine receptor (AR) subtypes, termed A₁, A_2A, A_2B and A₃, which belong to the G protein-coupled receptor (GPCR) superfamily. The human A₃AR (hA₃AR) subtype is implicated in several cytoprotective functions. Therefore, hA₃AR modulators, and in particular agonists, are sought for their potential application as anti-inflammatory, anticancer, and cardioprotective agents. Structure-based molecular modeling techniques have been applied over the years to rationalize the structure–activity relationships (SARs) of newly emerged A₃AR ligands, guide the subsequent lead optimization, and interpret site-directed mutagenesis (SDM) data from a molecular perspective. In this review, we showcase selected modeling-based and guided strategies that were applied to elucidate the binding of agonists to the A₃AR and discuss the challenges associated with an accurate prediction of the receptor extracellular vestibule through homology modeling from the available X-ray templates.

Scaffold-Hopping of Aurones: 2-Arylideneimidazo[1,2-a]pyridinones as Topoisomerase IIα-Inhibiting Anticancer Agents

Scaffold-hopping of bioactive natural product aurones has been studied for the first time. 2-Arylideneimidazo[1,2-a]pyridinones as potential topoisomerase IIα (hTopoIIα)-targeting anticancer compounds were considered. A multifunctional activator, polyphosphoric acid, enabled to realize a cascade reaction of 2-aminopyridine with 2,3-epoxyesters toward synthesis of 2-arylideneimidazo[1,2-a]pyridinones. Most of the compounds exhibited hTopoIIα-selective poison activity with efficiency more than etoposide and DNA-binding property, while not interacting with hTopo I. The compounds showed pronounced antiproliferative activities in nanomolar range with relatively poor toxicity to normal cells, inhibition of invasiveness, and apoptotic effect. The activities for inhibition of tubulin assembly, CDK1 and pCDK1, were also observed. Interestingly, the hTopoIIα inhibitory (in vitro and ex vivo studies) and antiproliferative activities of representative potent compounds were found to be manifold higher compared to corresponding parent aurones bearing alike substitutions, indicating the importance of such scaffold-hopping strategy in medicinal chemistry research.

Structure-Based Screening of Uncharted Chemical Space for Atypical Adenosine Receptor Agonists

Small molecule screening libraries cover only a small fraction of the astronomical number of possible drug-like compounds, limiting the success of ligand discovery efforts. Computational screening of virtual libraries representing unexplored chemical space could potentially bridge this gap. Drug development for adenosine receptors (ARs) as targets for inflammation and cardiovascular diseases has been hampered by the paucity of agonist scaffolds. To identify novel AR agonists, a virtual library of synthetically tractable nucleosides with alternative bases was generated and structure-based virtual screening guided selection of compounds for synthesis. Pharmacological assays were carried out at three AR subtypes for 13 ribosides. Nine compounds displayed significant activity at the ARs, and several of these represented atypical agonist scaffolds. The discovered ligands also provided insights into receptor activation and revealed unknown interactions of endogenous and clinical compounds with the ARs. The results demonstrate that virtual compound databases provide access to bioactive matter from regions of chemical space that are sparsely populated in commercial libraries, an approach transferrable to numerous drug targets.

Novel homologated-apio adenosine derivatives as A3 adenosine receptor agonists: design, synthesis and molecular docking studies

A series of homologated-apio adenosine derivatives including homologated-apio IB-MECA and Cl-IB-MECA have been designed and synthesized successfully. The molecular modelling and docking studies of the compounds have been explored as A₃AR agonists.

Tactical Approaches to Interconverting GPCR Agonists and Antagonists

There are many reported examples of small structural modifications to GPCR-targeted ligands leading to major changes in their functional activity, converting agonists into antagonists or vice versa. These shifts in functional activity are often accompanied by negligible changes in binding affinity. The current perspective focuses on outlining and analyzing various approaches that have been used to interconvert GPCR agonists, partial agonists, and antagonists in order to achieve the intended functional activity at a GPCR of therapeutic interest. An improved understanding of specific structural modifications that are likely to alter the functional activity of a GPCR ligand may be of use to researchers designing GPCR-targeted drugs and/or probe compounds, specifically in cases where a particular ligand exhibits good potency but not the preferred functional activity at the GPCR of choice.

5'-C-Ethyl-tetrazolyl-N(6)-substituted adenosine and 2-chloro-adenosine derivatives as highly potent dual acting A1 adenosine receptor agonists and A3 adenosine receptor antagonists

A series of N(6)-substituted-5'-C-(2-ethyl-2H-tetrazol-5-yl)-adenosine and 2-chloro-adenosine derivatives was synthesized as novel, highly potent dual acting hA1AR agonists and hA3AR antagonists, potentially useful in the treatment of glaucoma and other diseases. The best affinity and selectivity profiles were achieved by N(6)-substitution with a 2-fluoro-4-chloro-phenyl- or a methyl- group. Through an in silico receptor-driven approach, the molecular bases of the hA1- and hA3AR recognition and activation of this series of 5'-C-ethyl-tetrazolyl derivatives were explained.

Diazaspiro-iminosugars and polyhydroxylated spiro-bislactams: synthesis, glycosidase inhibition and molecular docking studies

Synthesis of a new class of iminosugars 1–4 has been reported.

New antitumor 6-chloropurine nucleosides inducing apoptosis and G2/M cell cycle arrest

Treating cancer has been challenging for decades, following countless approaches and attempts. Nucleosides, alone or as part of nucleotides, are vital elements of living systems and have shown pharmacological effects, e.g. as antibiotic or antiviral agents. We investigated the antitumor potential on human melanoma, lung and ovarian carcinomas, and on colon adenocarcinoma of a new series of purine nucleosides based on a 6-chloropurine or a 2-acetamido-6-chloropurine scaffold linked to perbenzylated hexosyl (glucosyl, galactosyl and mannosyl) residues. All compounds were tested in a sulforhodamine B (SRB) assay for their cytotoxicity and provided micromolar GI50 values with order of magnitude comparable to structurally similar chemotherapeutics, namely 2-chloro-2'-deoxyadenosine (cladribine). Furthermore, the induction of apoptosis was established and cell cycle analysis was accomplished demonstrating a G2/M cell cycle arrest.

Probing biased/partial agonism at the G protein-coupled A(2B) adenosine receptor

G protein-coupled A(2B) adenosine receptor (AR) regulates numerous important physiological functions, but its activation by diverse A(2B)AR agonists is poorly profiled. We probed potential partial and/or biased agonism in cell lines expressing variable levels of endogenous or recombinant A(2B)AR. In cAMP accumulation assays, both 5'-substituted NECA and C2-substituted MRS3997 are full agonists. However, only 5'-substituted adenosine analogs are full agonists in calcium mobilization, ERK1/2 phosphorylation and β-arrestin translocation. A(2B)AR overexpression in HEK293 cells markedly increased the agonist potency and maximum effect in cAMP accumulation, but less in calcium and ERK1/2. A(2B)AR siRNA silencing was more effective in reducing the maximum cAMP effect of non-nucleoside agonist BAY60-6583 than NECA's. A quantitative 'operational model' characterized C2-substituted MRS3997 as either balanced (cAMP accumulation, ERK1/2) or strongly biased agonist (against calcium, β-arrestin). N⁶-substitution biased against ERK1/2 (weakly) and calcium and β-arrestin (strongly) pathways. BAY60-6583 is ERK1/2-biased, suggesting a mechanism distinct from adenosine derivatives. BAY60-6583, as A(2B)AR antagonist in MIN-6 mouse pancreatic β cells expressing low A(2B)AR levels, induced insulin release. This is the first relatively systematic study of structure-efficacy relationships of this emerging drug target.

Microwave-assisted synthesis of novel purine nucleosides as selective cholinesterase inhibitors

A series of new purine nucleosides bearingd-glucosyl,d-mannosyl andd-galactosyl groups was synthesized. Selective inhibition of butyryl- or acetylcholinesterase could be tuned by purine substitution and glycosyl moiety. The 2-acetamidopurine nucleoside exhibiting ad-mannosyl group, α-configuration and N⁷-ligation was the most active and selective competitive inhibitor of butyrylcholinesterase.

Structure-based approaches to ligands for G-protein-coupled adenosine and P2Y receptors, from small molecules to nanoconjugates

Adenosine receptor (ARs) and P2Y receptors (P2YRs) that respond to extracellular nucleosides/nucleotides are associated with new directions for therapeutics. The X-ray structures of the A2AAR complexes with agonists and antagonists are examined in relationship to the G-protein-coupled receptor (GPCR) superfamily and applied to drug discovery. Much of the data on AR ligand structure from early SAR studies now are explainable from the A2AAR X-ray crystallography. The ligand-receptor interactions in related GPCR complexes can be identified by means of modeling approaches, e.g., molecular docking. Thus, molecular recognition in binding and activation processes has been studied effectively using homology modeling and applied to ligand design. Virtual screening has yielded new nonnucleoside AR antagonists, and existing ligands have been improved with knowledge of the receptor interactions. New agonists are being explored for central nervous system and peripheral therapeutics based on in vivo activity, such as chronic neuropathic pain. Ligands for receptors more distantly related to the X-ray template, i.e., P2YRs, have been introduced and are mainly used as pharmacological tools for elucidating the physiological role of extracellular nucleotides. Other ligand tools for drug discovery include fluorescent probes, radioactive probes, multivalent probes, and functionalized nanoparticles.

Structural sweet spot for A1 adenosine receptor activation by truncated (N)-methanocarba nucleosides: receptor docking and potent anticonvulsant activity

A(1) adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N(6)-cycloalkylmethyl 4'-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N(6)-dicyclopropylmethyl, K(i) = 47.9 nM) as a moderately A(1)AR-selective full agonist. Two stereochemically defined N(6)-methynyl group substituents displayed narrow SAR; groups larger than cyclobutyl greatly reduced AR affinity, and those larger or smaller than cyclopropyl reduced A(1)AR selectivity. Nucleoside docking to A(1)AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger "A" forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39), and carbon chains of glutamates (EL2) and the smaller subpocket "B" forming contacts between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A(1)AR agonists. Truncated nucleosides, an appealing preclinical approach, have more druglike physicochemical properties than other A(1)AR agonists. Thus, we identified highly restricted regions for substitution around N(6) suitable for an A(1)AR agonist with anticonvulsant activity.

Structure-guided design of A(3) adenosine receptor-selective nucleosides: combination of 2-arylethynyl and bicyclo[3.1.0]hexane substitutions

(N)-Methanocarba adenosine 5'-methyluronamides containing known A(3) AR (adenosine receptor)-enhancing modifications, i.e., 2-(arylethynyl)adenine and N(6)-methyl or N(6)-(3-substituted-benzyl), were nanomolar full agonists of human (h) A(3)AR and highly selective (K(i) ∼0.6 nM, N(6)-methyl 2-(halophenylethynyl) analogues 13 and 14). Combined 2-arylethynyl-N(6)-3-chlorobenzyl substitutions preserved A(3)AR affinity/selectivity in the (N)-methanocarba series (e.g., 3,4-difluoro full agonist MRS5698 31, K(i) 3 nM, human and mouse A(3)) better than that for ribosides. Polyaromatic 2-ethynyl N(6)-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (K(i) hA(3) 3.1 nM; A(1), A(2A), inactive) and fluorescent 1-pyrene adduct MRS5704 35 (K(i) hA(3) 68.3 nM), were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A(2A)AR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, the X-ray structure of related A(2A)AR is useful in guiding the design of new A(3)AR agonists.

Click modification in the N6 region of A3 adenosine receptor-selective carbocyclic nucleosides for dendrimeric tethering that preserves pharmacophore recognition

Adenosine derivatives were modified with alkynyl groups on N(6) substituents for linkage to carriers using Cu(I)-catalyzed click chemistry. Two parallel series, both containing a rigid North-methanocarba (bicyclo[3.1.0]hexane) ring system in place of ribose, behaved as A(3) adenosine receptor (AR) agonists: (5'-methyluronamides) or partial agonists (4'-truncated). Terminal alkynyl groups on a chain at the 3 position of a N(6)-benzyl group or simply through a N(6)-propargyl group were coupled to azido derivatives, which included both small molecules and G4 (fourth-generation) multivalent poly(amidoamine) (PAMAM) dendrimers, to form 1,2,3-triazolyl linkers. The small molecular triazoles probed the tolerance in A(3)AR binding of distal, sterically bulky groups such as 1-adamantyl. Terminal 4-fluoro-3-nitrophenyl groups anticipated nucleophilic substitution for chain extension and (18)F radiolabeling. N(6)-(4-Fluoro-3-nitrophenyl)-triazolylmethyl derivative 32 displayed a K(i) of 9.1 nM at A(3)AR with ∼1000-fold subtype selectivity. Multivalent conjugates additionally containing click-linked water-solubilizing polyethylene glycol groups potently activated A(3)AR in the 5'-methyluronamide, but not 4' truncated series. N(6)-Benzyl nucleoside conjugate 43 (apparent K(i) 24 nM) maintained binding affinity of the monomer better than a N(6)-triazolylmethyl derivative. Thus, the N(6) region of 5'-methyluronamide derivatives, as modeled in receptor docking, is suitable for functionalization and tethering by click chemistry to achieve high A(3)AR agonist affinity and enhanced selectivity.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Truncated (N)-Methanocarba Nucleosides as A(1) Adenosine Receptor Agonists and Partial Agonists: Overcoming Lack of a Recognition Element

A(1) adenosine receptor (AR) agonists are neuroprotective, cardioprotective, and anxiolytic. (N)-Methanocarba adenine nucleosides designed to bind to human A(1)AR were truncated to eliminate 5'-CH(2)OH. This modification previously converted A(3)AR agonists into antagonists, but the comparable effect at A(1)AR is unknown. In comparison to ribosides, affinity at the A(1)AR was less well preserved than at the A(3)AR, although a few derivatives were moderately A(1)AR selective, notably full agonist 21 (N(6)-dicyclopropylmethyl, K(i) 47.9 nM). Thus, at the A(1)AR recognition elements for nucleoside binding depend more on 5'region interactions, and in their absence A(3)AR selectivity predominates. Based on the recently reported agonist-bound AR structure, this difference between subtypes likely correlates with an essential His residue in transmembrane domain 6 of A(1) but not A(3)AR. The derivatives ranged from partial to full agonists in A(1)AR-mediated adenylate cyclase inhibition. Truncated derivatives have more drug-like physical properties than other A(1)AR agonists; this approach is appealing for preclinical development.

New 2-heterocyclyl-imidazo[2,1-i]purin-5-one derivatives as potent and selective human A3 adenosine receptor antagonists

A series of 4-allyl/benzyl-7,8-dihydro-8-methyl/ethyl-2-[(substituted)isoxazol/pyrazol-3/5-yl]-1H-imidazo[2,1-i]purin-5(4H)-ones has been synthesized and evaluated in radioligand binding assays to determine their affinities at the human A(1), A(2A), and A(3) adenosine receptors. Efficacy at the hA(2B) AR and antagonism of selected ligands at the hA(3) AR were also assessed through cAMP experiments. All of the synthesized molecules exhibited high affinity at the hA(3) AR (K(i) values ranging from 1.46 to 44.8 nM), as well as remarkable selectivity versus A(1), A(2A), and A(2B) AR subtypes. Compound (R)-4-allyl-8-ethyl-7,8-dihydro-2-(3-methoxy-1-methyl-1H-pyrazol-5-yl)-1H-imidazo[2,1-i]purin-5(4H)-one (R-33) was found to be the most potent and selective ligand of the series (K(i) hA(3) = 1.46 nM, K(i) hA(2A)/K(i) hA(3) > 3425; IC(50) hA(2B)/K(i) hA(3) > 3425; K(i) hA(1)/K(i) hA(3) = 1729). Molecular modeling studies were helpful in rationalizing the available structure-activity relationships along with the selectivity profiles of the new series of ligands.

Functionally biased modulation of A(3) adenosine receptor agonist efficacy and potency by imidazoquinolinamine allosteric enhancers

Allosteric modulators for the G(i)-coupled A(3) adenosine receptor (AR) are of considerable interest as therapeutic agents and as pharmacological tools to probe various signaling pathways. In this study, we initially characterized the effects of several imidazoquinolinamine allosteric modulators (LUF5999, LUF6000 and LUF6001) on the human A(3) AR stably expressed in CHO cells using a cyclic AMP functional assay. These modulators were found to affect efficacy and potency of the agonist Cl-IB-MECA differently. LUF5999 (2-cyclobutyl derivative) enhanced efficacy but decreased potency. LUF6000 (2-cyclohexyl derivative) enhanced efficacy without affecting potency. LUF6001 (2-H derivative) decreased both efficacy and potency. We further compared the agonist enhancing effects of LUF6000 in several other A(3) AR-mediated events. It was shown that although LUF6000 behaved somewhat differently in various signaling pathways, it was more effective in enhancing the effects of low-efficacy than of high-efficacy agonists. In an assay of cyclic AMP accumulation, LUF6000 enhanced the efficacy of all agonists examined, but in the membrane hyperpolarization assay, it only enhanced the efficacy of partial agonists. In calcium mobilization, LUF6000 did not affect the efficacy of the full agonist NECA but was able to switch the nucleoside antagonist MRS542 into a partial agonist. In translocation of β-arrestin2, the agonist-enhancing effect LUF6000 was not pronounced. In an assay of ERK1/2 phosphorylation LUF6000 did not show any effect on the efficacy of Cl-IB-MECA. The differential effects of LUF6000 on the efficacy and potency of the agonist Cl-IB-MECA in various signaling pathway were interpreted quantitatively using a mathematical model.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Synthesis of 2-chloro-N6-substituted-4'-thioadenosine-5'-N, N-dialkyluronamides as potent and selective A3 adenosine receptor antagonists

The highly selective A(3) receptor agonist, 4'-thio-Cl-IB-MECA was successfully converted into selective A(3) receptor antagonists by appending a second N-alkyl group on the 5'-uronamide position. This result indicates that the hydrogen bonding ability of the 5'-uronamide is essential for the conformational change required for the receptor activation. Among compounds tested, a N(6)-(3-bromobenzyl) derivative with 5'-dimethyluronamide exhibited the highest binding affinity (K(i) = 9.32 nM) at the human A(3) AR with very low binding affinities to other AR subtypes.